If you add a value in session variable and again you have add another value in same session variable how to retrieve both value from this variable

You can't store multiple objects in a Session variable. If you want to store more than one thing you'll need to use a List or similar

var basket = new List<Product>();
basket.Add(new Product { ... });
basket.Add(new Product { ... });
Session["Basket"] = basket;

So if you only have one thing in the basket then it will still be a list but with only one item.